1. Field of the Invention
The invention relates generally to an improved fiber optic system in which the transmission of light along a fiber optic core of the system is stabilized and optimized. More particularly, the invention provides an apparatus and method which eliminates the destabilizing effect of light propagated in a clad surrounding the fiber optic core of a fiber optic element, by preventing light from being directly input from a photoemitter to the clad and/or by preventing light from being transmitted from the clad to a photodetector. The improved fiber optic system according to the invention further provides a means and apparatus for controlling the light input from the photoemitter to the fiber optic core by providing an aperture stop means between the photoemitter and the fiber optic element.
The terminology "fiber optic system" as used herein refers to a system of the type having a fiber optic element including a fiber optic core made of highly light-transmissive optical fiber for transmitting light from a photoemitter to a photodetector. The fiber optic element further includes a clad surrounding the fiber optic core, the clad being made of material having a lower refractive index than that of the core. Typically, the photoemitter takes the form of a light-emitting diode (LED). Light input to the fiber optic core by the LED traverses the core and becomes incident on the photodetector, which converts the modulated light into an electrical output signal which may be amplified for recording data.
The terminology "fiber optic sensor" as used herein refers to any one of a variety of types of fiber optic sensors, including one in which at least a portion of the clad of the fiber optic element is made of a material responsive to an analyte of interest. The clad material is responsive to a given chemical(s) or other analyte, such that its refractive index changes in the presence of such analyte(s) to thereby alter the light-transmitting properties of the sensor.
The term "analyte" as used herein refers to a chemical or physical property desired to be detected and/or measured in a medium of interest. By way of example, an analyte may refer to a given chemical or class of chemicals, a chemical compound or class of chemical compounds, water, biological agents, pressure, temperature, and the like.
2. Description of Relevant Art
There are generally known numerous types of fiber optic sensors for detecting and/or measuring an analyte of interest by employing the inherent light-transmission properties of an optical fiber. Illustrative of a known type of fiber optic sensor is the "Fiber Optic which is an Inherent Chemical Sensor" disclosed in U.S. Pat. No. 4,846,548 issued in 1989 to Klainer.
With reference to FIG. 1, there is shown a typical fiber optic sensor of the type having an analyte-responsive clad. The sensor 1 has a fiber optic core 2 constructed of a short length of glass fiber, such as silica fiber, having a diameter of approximately 1 mm or less and a length of approximately 50 mm or less. Surrounding the core is a clad 3 including a portion 3A made of a sensing material which is responsive to a chemical or analyte of interest, the clad 3, 3A typically having a depth of approximately 20 microns or less. The core 2 has a refractive index which is greater than the refractive index of the clad 3. As such, when light is input to the fiber optic core 2 from a light source or LED 5, light which is incident on the core-clad boundary interface 4 at a critical angle or greater (described below) will be totally internally reflected and transmitted along the core 2. However, when the sensor is in the presence of a chemical or analyte to which the clad portion 3A is particularly adapted to be responsive, the refractive index of clad portion 3A changes so that the amount and angles of light internally reflected from the core-clad interface 4 also changes. The resulting change in light transmission along core 2 in turn results in a change in the intensity and/or angle of light signals detected by photodetector 6, which change may be correlated to a known relationship between the chemical or analyte of interest and the clad.
The present inventors, in conducting numerous experiments with fiber optic sensors having analyte-sensitive clads, encountered stability and reliability problems which have generally plagued others in the field. The inventors' repeated testing of fiber optic sensors yielded unsatisfactory and inconsistent data due to the instability of light transmission along the fiber optic core. The causes of these problems have eluded those skilled in the art since the advent of this type of sensor. Although many attempts have been made to solve the problem through the use of various electronic arrangements and the like, the attempts have proven unsuccessful.
The present invention eliminates the stability and reliability problems associated with unstable light transmission along the core of a fiber optic sensor. To effectively stabilize core light transmission, the invention provides a method and apparatus for eliminating or controlling the independent propagation of light in the clad surrounding the core ("clad light"). As shown in FIG. 1, the input of light to the fiber optic sensor in known designs has been indiscriminate, with light from LED 5 being input to both the fiber optic core and the clad 3. The light input to the clad propagates along the clad, and is very sensitive to a host of factors. The inventors discovered that, apparently due in part to the short length of the fiber optic element in a fiber optic sensor, this independent clad light has a highly destabilizing effect on the transmission of light through the fiber optic core. The effect of this clad light has been to introduce sufficient "noise" as to substantially interfere with the light transmitted through the fiber optic core.
The destabilizing effect of clad light in fiber optic systems, as discovered by the present inventors, may be understood with reference to FIG. 1. The phenomenon of total internal reflection, upon which the fiber optic sensor relies for its basic functioning, is indicated by the ray of light 10 input to the fiber optic core 2 by the LED 5. Total internal reflection occurs by virtue of the relatively low refractive index N.sub.2 of the clad in comparison with the relatively high refractive index N.sub.1 of the core. Light rays input to core 2 which are incident on the interface 4 between the core 2 and the clad 3 at a critical angle or greater will be totally reflected. The critical angle, measured from the normal to the surface at which light enters the clad, is: EQU A.sub.c =sin.sup.-1 N.sub.2 /N.sub.1
In FIG. 1, ray 10 from LED 5 is incident on the interface 4 at an angle which is greater than the critical angle A.sub.c, so that ray 10 is totally internally reflected within the core.
In FIG. 1, the destabilizing effect of clad light in the fiber optic chemical sensor is indicated by the ray of light 11 input to the clad 3 by the LED 5. As shown in FIG. 1, light from LED 5 has heretofore been indiscriminately input to both the core and the clad of the fiber optic element, without any appreciation of the problems generated thereby. Because light is input to the clad, the phenomenon of total internal reflection, upon which the fiber optic sensor relies for its basic functioning in the transmission of light through the fiber optic core, also tends to occur in the clad. This total internal reflection effect within the clad occurs whenever the ambient medium surrounding clad 3 has a lower refractive index than that of the clad, i.e., lower than N.sub.2. The phenomenon of total internal reflection of light ray 11 input to clad 3 occurs at the interface 8 between the clad and the ambient in essentially the same way as described above with respect to the interface 4 between the core 2 and the clad 3. Although the critical angle in clad 3 differs from the critical angle A.sub.c described above, and varies along clad portion 3A when the refractive index thereof changes in the presence of an analyte, light rays incident on interface 8 between the clad 3 and a surrounding medium having a lower refractive index than N.sub.2, at an angle equal to greater than the relevant critical angle, will be totally reflected. Thus, ray 11 incident on interface 8 at an angle greater than the critical angle is totally internally reflected within the clad such that light independently propagates along the clad.
Even if the medium surrounding the clad 3 has a higher refractive index than the refractive index N.sub.2 of the clad, so that the phenomenon of total internal reflection does not occur, some amount of light input to the clad will be reflected back into the clad at both interfaces 4 and 8, and propagated along the clad. Further, although the core 2 has a higher refractive index N.sub.1 than the refractive index N.sub.2 of the clad, so that no total internal reflection occurs at interface 4 with respect to the independently-propagated clad light, some amount of this clad light is reflected back into the clad at interface 4, while some amount of light is refracted through core 2. It will thus be understood that the light independently propagated along the clad is not limited to the simplistic zigzag pattern which is shown in FIG. 1 for ease of illustration, but rather will also in part be refracted into core 2 at interface 4.
The present inventors discovered that the foregoing clad light phenomenon, i.e., the reflection and refraction of light rays input to the clad 3 by photodetector 5 and propagated along the clad, produces sufficient "noise" as to interfere with the transmission of light through the fiber optic core and destabilize same. The deleterious effect of clad light on the stability and reliability of fiber optic systems which may be sensitive thereto, such as the effect of same on fiber optic sensors, has heretofore eluded those skilled in the art. As a consequence, for example, fiber optic sensors of the type having an analyte-reactive clad have heretofore proven so unstable and unreliable as to be incapable of functioning effectively to detect and/or measure analytes in a medium of interest.
The present invention eliminates the above-described destabilizing effect of clad light on the transmission of light through, and/or emission of light from, the fiber optic core by employing a field stop and/or clad light stop means suitably arranged relative to the core, the clad, and the LED or photodetector, of a fiber optic system. The invention thus vastly improves the reliability of the system, particularly where it takes the form of a fiber optic sensor, by stabilizing emitted signals and temperature conditions, and enhancing the sensor's resistance to stress.
In addition to eliminating the destabilizing effect of clad light, the invention further optimizes the transmission of light through the fiber optic core of a fiber optic system by providing an aperture stop means between the LED and the fiber optic core. Heretofore, fiber optic cores such as those employed in fiber optic sensors have exhibited considerable variability with respect to initial output signals, i.e., signals detected by the photodetector. Prior to compensation or amplification, output signals vary considerably from one fiber optic element to the next. In a system where an optimal initial output signal is approximately one volt, for example, various compensating and/or amplifying steps may be required in order to obtain the desired initial output signal from a given fiber optic element. To eliminate these problems, the present invention provides an aperture stop means between the LED and the fiber optic element, so as to focus the conical radiation pattern from the LED onto the fiber optic core. By varying the aperture size of the aperture stop means, the variability in initial output signals from one fiber optic element to the next can be effectively compensated for and eliminated.